“Number rules the universe.” – Pythagoras
Risk Priority Number (RPN) . . . by definition, a single number that represents relative risk for prioritizing issues in an FMEA. Sounds simple. But, how well does it work in practice? And, is there a better way?
This article highlights shortcomings and concerns when using RPN in FMEA, and discusses an alternative. It is a companion article to “Prioritizing risk for corrective actions in an FMEA.”
Refresh: What is RPN?
As covered in the above referenced article:
In an FMEA, Risk Priority Number (RPN) is a numerical ranking of the risk of each potential failure mode and associated cause, made up of the arithmetic product of the three elements: severity of the effect, likelihood of occurrence of the cause, and likelihood of detection of the cause.
How well does RPN prioritize risk?
The following are the primary shortcomings and concerns when using RPN.
1. High severity – low RPN can be high risk. Therefore, if you use RPN as the primary method to prioritize risk, high-severity issues can be under-represented. The work around is to always address high-severity first, regardless of RPN. Safety and regulatory risk must be addressed by first attempting to reduce severity (if possible), followed by driving occurrence and detection risk as low as possible. High-risk issues should always be reviewed with management.
2. RPN thresholds are not advised. They can devolve into a numbers game. International FMEA standards have discouraged this practice for years.
3. RPN has been replaced by “Risk Prioritization” in international FMEA standards. The new AIAG/VDA FMEA Handbook does not use RPN. The new SAE J1739 FMEA Standard considers RPN as one of three possible approaches to risk prioritization.
Is there a better way?
The objective of Risk Prioritization is to sequence the risk associated with each of the failure modes, effects and causes to determine which are highest priority for corrective actions. One alternative worth considering is called Action Priority (AP).
What is Action Priority (AP)?
The Action Priority (AP) method of risk prioritization accounts for all combinations of S, O, and D. This allows more emphasis on severity, and adjusts for the shortcoming in RPN. The combinations for S, O, and D are represented in an Action Priority Table.
According to SAE J1739 (2021):
Action Priority (AP) table combines Severity (S), Occurrence (O), and Detection (D) ratings with suggested priority levels for identification of potential actions to reduce risk based on High (H), Medium (M), and Low (L) assessments. Individual companies may develop their own criteria for H, M, and L and make the company-specific Action Priority table available to the DFMEA team and customer or supplier.
The benefit of action priority is that it does not treat severity, occurrence, and detection as equal values (as RPN, SO does). The AP table provides an action prioritization system to focus the team’s time and resources.
Management can provide guidance and should be kept informed on all high-risk issues.
What does an AP Table look like?
Here is an example AP Table. Note, in this example, S, O, and D scales are 1 to 10. The same type of table can be done with other scales, such as 1 to 5.
The AP Table should be company-specific, based on the risk considerations of the company.
How is AP Table applied?
In an FMEA, as soon as the FMEA team assesses the risk associated with Severity, Occurrence and Detection, the next step is Risk Prioritization. The FMEA team finds the applicable column in the AP Table, and identifies the corresponding AP value. For example, if S = 8, O = 3, and D = 5, the AP in the above table would be “M.” This means the team would first address all of failures / causes that are “H,” before considering action for failures / causes that are “M” or “L.”
As noted in SAE J1739, “It is recommended that potential Severity 9-10 and potential effect(s) regardless of Action Priority assessment be reviewed by management including any recommended actions that were identified.” [The same note applies to Severity 5, when using a 1 to 5 Severity scale.]
An AP assessment of Low does not mean action should not be considered. The High, Medium or Low assessment should be used to prioritize action, not presume action is not necessary.
This concludes the Inside FMEA “Special Topics” series of articles. Next month will begin a new series called “Current FMEA Topics.” The first discussion topic will be “FMEA: Automated or Team-Based.”
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